I'm currently working on a film where we are doing many 3D projection shots.
Here's how the workflow is happening....

3D renders a still from the projection camera(s) for the matte painters.
These files are 32 bit EXR's. We open them in photoshop and convert them
to 16 bit. Paint paint paint. Finally, we convert them back to 32 bit EXR's
for reprojection and compositing.

BUT, I learned something quite valuable. If you are converting from 16 bit
to 32 bit, you will have problems with your layer transparency, especially
if you have layers set to anything less than 100% opacity. Changing to
32 bit will change the appearance of your layers unless-

You go to the edit menu. Choose 'edit>color settings'
When the dialogue box comes up, choose 'more options'
The last checkbox on the bottom should be 'Blend RGB Colors using Gamma 1.0".
Check this box.

Now, when you convert your layered Photoshop file to 32 bit, the appearance of your layers will stay the way you want.

In order to save your file with layers intact (handy for sending separate
cloud layers to another program, for example) you will need a 3rd party
plugin, as Adobe's implementation of OpenEXR is very basic. The only
plugin I know of that will open and save layered EXR files is 'ProEXR'.
I have it and it works.